Eye diseases and conditions can be treated with topical agents, such as drops or ointments, which use direct absorption to reach therapeutic drug levels at the target tissue. Topical application is especially useful for the cornea, which has virtually no blood supply and is easily accessible for topical application. When drugs are applied topically in the form of drops or ointment to the cornea, however, the drug solution rapidly disperses into the tear film and flows into the tear drainage system, thereby reducing bioavailability. Non-limiting examples of factors affecting the bioavailability of a drug (i.e., drug levels) in the cornea and to the anterior chamber of the eye include the duration of the drug's effective concentration at the surface of the cornea, and the drug's ability to be absorbed into the corneal tissue.
Delivering medications in the form of eye drops can have two main disadvantages. First, the concentration of the drug can rapidly fall below therapeutic levels, potentially requiring frequent applications of the drug. Second, high concentrations of the drug may be necessary to achieve adequate penetration into the eye because of the drug's brief contact time with the eye, and these high concentrations may be toxic.
An existing method of delivering a drug to the cornea involves dehydrating a soft contact lens, then soaking the lens in a solution of the drug. This method provides an initial burst release of the drug followed by continuous decline in the amount of drug at the cornea. This leads to limited bioavailability of the drug after the initial burst release.
Another method of delivering a drug to the cornea involves soaking a collagen shield in a solution of the drug. These collagen shields are opaque and must be worn in a recess under the lids. Because the impregnated drug is carried to the corneal surface by the tears, its bioavailability is limited in dry eye disease, a common condition in eyes with neuropathic pain.